Washing the car regularly is a good way to protect the undercarriage of the car from the corrosion of salt and other forms of buildup. Applying an undercarriage seal is an effective step, particularly in areas where salt is used as a frequent road treatment.
Salt has a way of building up underneath the car in places that are not visible to the naked eye. When it is wintertime, washing the car every 10 days keeps the salt from aggregating. It's best to wash it during the day so that the water does not freeze to the car when it cools down at night.
Driving over big piles of snow can send concealed salt up into the undercarriage as the snow often conceals a big pile of salt. Salt also tends to collect in big puddles, so steering around those also helps to keep salt from building up on the underside of the car.
Put a seal on the undercarriage of the car. These are available for purchase, and you can install them yourself or have them installed by a service company. Because your car is a major investment, a seal like this is worthwhile in parts of the country where roads freeze frequently.